Hard case for airguns

Depending on what you're really looking for, AoA sells some very nice Boyt cases that are airline approved, solid as a gun safe, and you custom cut the interior for your gun. I have a couple of them. I'm also a big fan of the aluminum molded Halliburton cases. My sniper rifle has traveled in one for decades, but I'm not sure they're still available.

Someone mentioned Harbor Freight which would be a good option. As much as I hate Wmart, in the past they have had some hard plastic that were just dirt inexpensive. I bought about 25 of them once to transport several firearms.